Originally Posted by ackattacker

That is 100% correct. For an aircraft at a given weight, steady state flight, increases in altitude will decrease Mcrit for exactly the reason you stated.

Keep in mind Mcrit is not a limitation like Mmo. Most jet aircraft cruise around .02 or .03 faster than Mcrit, which is just the "onset" of mach effects such as tuck and wave drag. The faster you go past Mcrit, the more those effects begin to be felt until eventually the designers impose the Mmo.